In methods of assembling parts together, in particular automotive bodywork parts, it is common practice to bring the various parts for assembly to an assembly station where they are firstly put accurately into position relative to one another, and secondly they are held in position firmly so as to ensure that any forces applied to them by the assembly tooling (welding clamps in particular) do not disturb, alter, or modify their intended positions and thus the general shape of the assembly once it has been made.
An assembly station is fed with parts for connecting to one another by a handling tool (a robot) which xe2x80x9centrustsxe2x80x9d the part it is transporting to tooling specific to the assembly station, which tool then ensures that the part is put accurately into place and is clamped firmly in place. That conventional disposition suffers from the major drawback of requiring the assembly station to implement tools that are dedicated to each of the parts received. Consequently, to switch from manufacturing one product to another, although it is possible to retain a single handling robot, it is necessary to change the tooling for positioning and clamping the parts relative to the assembly station. That requirement constitutes a major limit on assembly station flexibility, i.e. on its capacity to receive different types of part. It is necessary to have one set of tools for each part and to fit the station with tool-changing means so that such a changeover can be performed as quickly as possible. The assembly station then becomes very cluttered by means for handling these sets of tools (which are often presented in the form of prefitted pallets. It is also necessary for the handling means to be manufactured with care so that the sets of tools are put into place as accurately as possible within the station and do not give rise to unacceptable dispersion in the accuracy with which the parts for assembling together are put into place. Furthermore, such tool changes still require time and that constitutes a factor for lengthening assembly cycle times, which necessarily leads to increased production costs.
Nowadays, the accuracy of handling robots concerning control over the paths they follow and the coordinates of the starting and ending positions of each such path are becoming entirely compatible with the accuracy required for positioning parts in the frame of reference of an assembly station. As a result, it is possible merely by changing the programming of a handling robot to put various different parts accurately into place in an assembly frame of reference within which, likewise by suitable and varying programming, it is possible to give accurate positions to assembly tools such as welding clamps.
It is thus possible to eliminate the specific positioning tools that were previously necessary in assembly stations. Unfortunately, the structure of a robot is unsuitable for withstanding the forces to which parts are subjected by the action of the assembly tools in order to keep each part firmly in position during an assembly operation (e.g. by welding). This leads to a major risk of the resulting assembly having the wrong shape.
The present invention seeks to provide a solution for use in assembling parts (at least two parts) and in particular automotive bodywork parts, which takes advantage of the accuracy of robots to simplify considerably the tooling of assembly stations and thus make them suitable for accepting parts of different shapes, with the consequence of making such stations very flexible in use.
To this end, the invention provides a method of holding a part in position in an assembly station, in which method the part is put into a determined position in the frame of reference of the assembly station by means of a handling robot, at least one clamp is closed onto a portion secured to the part, the clamp being mounted to slide freely in a guide of the assembly station extending parallel to its own clamping direction, and the clamp is blocked against sliding when it is clamped onto the part.
In other words, in the method of the invention, once the part has been put into place by the positioning robot, the part is clamped onto a support which forms part of the assembly station while ensuring that the clamping forces do not constitute parasitic forces concerning the positioning of the part, given that the robot which holds the part in position is not capable of opposing such forces adequately. For example, it will be understood that a clamp which is mounted to slide on the above-mentioned support along a guide parallel to the clamping direction of its jaws cannot exert a force in the sliding direction. Consequently, in theory, clamping does not cause the part to move at all. Once the part has been gripped between the jaws, it suffices to lock a clamp in its guide to ensure that the part is clamped on the support without its position as defined by the robot being altered.
There are several ways in which the clamps can be locked, i.e. in which its degree of freedom along its guide can be eliminated. In a first embodiment, sliding is locked by clamping at least one second sliding clamp having a sliding direction that is not parallel to the sliding direction of the first clamp whose freedom to move along its guide is to be eliminated.
In a second embodiment, sliding is locked by a brake, which acts between the guide and the clamp-carrier slider.
Depending on the shape of the assembly to be made, the number of parts to be put into position relative to one another in the assembly station, the size of these parts, the holds which the handling robot must have on them, and the freedom of access that needs to be left for the assembly tools, the clamps for clamping the part on the support forming a portion of the assembly station act either directly on the part, or else they act on a portion of the robot close to its part-gripping end.